This subject matter disclosed herein relates generally to medical imaging systems, and more particularly, to Positron Emission Tomography (PET) systems, such as Time of Flight (TOF) PET systems.
A PET system generates images that represent the distribution of positron-emitting nuclides within the body of a patient. When a positron interacts with an electron by annihilation, the entire mass of the positron-electron pair is converted into two 511 keV photons. The photons are emitted in opposite directions along a line of response. The annihilation photons are detected by detectors that are placed along the line of response on a detector ring. When these photons arrive and are detected at the detector elements at the same time, this is referred to as coincidence. An image is then generated based on the acquired image data that includes the annihilation photon detection information.
In PET systems, the image quality depends on image statistics. The image statistics may be improved by acquiring the image data for longer durations. However, the total time required to acquire the image data is limited by the lifetime of the radioactive isotope used in the imaging process and by the inability of the patients to remain immobile for extended durations. Image quality may be improved by including TOF information of the emission data, which generally refers to the difference in the time at which the photons are detected by the detector elements. The timing difference is used to localize the source of emission along the line joining two detector elements in TOF PET systems.
In order to maintain a good signal-to-noise ratio in the images in the reconstruction process in TOF PET systems, these systems need to accurately calculate the timing difference. The timing capability of PET systems depends on different factors including the amount of “fast” light output from the scintillator and the quantum efficiency of photosensors of detectors of these systems, as well as geometrical factors, such as the transmission efficiency in scintillators of the detectors, light collection methods and efficiency, the size of the detectors, the reflective material used, and the refractive index of matching of the material used, among others.
With respect to the timing resolution of PET systems, the crystal size of the detectors affects the timing resolution due to the spread of gamma ray interaction points and the degree of scintillation light spread/loss inside a crystal. Smaller and more flat crystals are less sensitive to both types of spreads and can provide improved timing resolution. However, smaller crystals may not have enough stopping power to be used in particular detectors, such as whole body PET detectors.